1. Field of the Invention
The present invention relates to an image pickup optical system and an image pickup apparatus for separating a part of a light beam traveling from a subject.
2. Description of the Related Art
One of passive autofocus systems of a camera or video camera is a pupil division method for phase detection autofocus. The pupil division method is a method of obtaining a focus signal. In this method, a part of a light beam from a subject is separated from the light beam by a light beam division element and made incident on an image pickup element for focus detection (U.S. Patent Application Publication No. 2003/0174232 and U.S. Patent Application Publication No. 2004/0036794).
Such method has a feature that a focus position may be searched without an influence on a pickup image, because a lens provided on an optical path of a separated light beam is not driven to search the focus position (detect focus position).